The present invention relates to a webbing sensor for a vehicle safety restraint retractor.
A webbing sensor is used to lock a retractor against pay out of seat belt webbing when it detects a sudden accelerating force on the webbing indicative of a sudden forward movement of the vehicle occupant such as would happen in an emergency situation.
Typically a webbing sensor is formed of an inertial disc or part of a disc mounted coaxially with the spool of a retractor and connected thereto by a spring. In normal use of the seat belt the disc will rotate with the spool as webbing is payed out or wound onto the spool. However a sudden change in the speed of rotation of the spool in the paying out direction results in a phase difference between the disc and the spool, i.e. the disc lags behind the spool. The disc is connected to a pawl and when this lag occurs it moves the pawl radially outwardly to engage teeth on the lockcup and thereby lock the spool against further rotation and hence against further pay out of webbing thus securing the vehicle occupant against further forward movement.
One problem with the sensor is that the resilience of the spring increases as it is compressed. This means that the locking action is not as quick and clean as would ideally be desired.
It is an object of the present invention to provide an improved webbing sensor.
According to the present invention there is provided a webbing sensor for a vehicle safety restraint retractor, the retractor comprising a rotatable spool on which seat belt webbing is wound, and means for biasing the spool in a webbing rewind direction, the sensor comprising:
an inertial member, mounted for rotation coaxially with the spool,
a pivotally mounted locking pawl for engaging the spool and locking it against rotation under emergency conditions,
a calibration spring biasing the inertial disc so that it tends to rotate with the spool unless there is a sudden acceleration or deceleration of a value such as to overcome the resilience of the spring,
wherein the spring is connected between the inertial member and the pawl.
According to a preferred embodiment the spring is connected so that relative rotation between the inertial member and the spool results in movement of both ends of the spring, and preferably by approximately the same distance.
Hence throughout the locking procedure, the spring force remains substantially constant and thus the bias torque on the inertia disc remains substantially constant and the locking performance of the sensor is improved. Additionally, with this geometry, the mass of the disc can be made smaller than in traditional sensors resulting in a production cost saving. A reduction of 20% in the mass of the disc results in only a 5% reduction in the inertia of the disc, because the mass of the disc can be concentrated at its periphery. This also allows use of plastics material instead of metal for the disc.
Preferably the locking pawl is pivotally mounted on the spool at a point offset from the axis of rotation of the spool and comprises a transversely extending lug adapted to engage in a channel formed in the inertial member. The spring may be located in a recess in the inertial member and the channel may extend off this recess.
According to another aspect of the invention, the sensor is constructed as an assembled unit which can be fitted to the retractor as a self contained sub-assembly. This is in contrast to the traditional sensor which is constructed in the retractor by the assembly of individual components during assembly of the retractor. Constructing the disc, spring and the pawl as a sub-assembly in this way greatly simplifies the production process and leads to cost savings. Previous designs had the spring anchored to the spool and thus could not be sub-assembled.